Heat Transfer

When air passes over an object, it can accumulate heat energy or deposit heat on the object. This is thermal convection in action. The radiator is designed to transfer the coolant's heat energy to the air. As air passes through the radiator, the heat in the coolant actually passes through the metal and is absorbed by the air. When the air reaches the far side of the radiator, it is at a greater temperature and the coolant is at a lower temperature because it dissipated its heat into the air. The same principle is at work within the transmission cooler of a radiator, if the car has an automatic transmission. The transmission fluid flowing through the cooler gives off it's heat to the coolant within the radiator, which then gives off it's heat to the air flowing through the radiator. Disc brakes and various other parts which need cooling dissipate heat directly into the air without using water. Some types of cars, such as Volkswagens, use air cooled engines. These eliminate the need for water by having more engine surface area through the use of cast fins. These allow the air to pass over a large surface area of the engine and thus transfer heat directly to the air. As your car travels down the road, the air that passes through the radiator grille either exits through the floor of the engine compartment, or it may pass out through the sides of the car, through what are known as gills. These side vents allow the air, which is compressed within the engine compartment to exit to an area which is of relatively low pressure rather than trying to force it under the car where there isn't as much room for it.